This invention relates to an adapter for installation in an AR-15 or M16-type rifle to change a one-stage trigger system to a two-stage trigger system. A two-stage trigger system eliminates what is commonly known as "creep" after initial take-up of trigger movement and gives the shooter an indication of eminent firing immediately prior to firing.
In a single-stage trigger system the trigger force is dependent on two disparate forces. The first is the torsional trigger spring force occurring due to rotation of the trigger about its pivot point, creating increased torque on the trigger spring that presses on the lower receiver. The second force is that due to frictional engagement between the trigger sear surface and the mating hammer sear surface, which are slidably engaged when the hammer is in the cocked position. The sear surface friction results from a torsional hammer spring that provides a rotational force to the hammer, which is restrained at the sear surfaces until the trigger releases the hammer. Typical weapons of the type disclosed herein provide a relatively uniform or constant trigger force of between 5 and 9 pounds during the discernible movement of the trigger prior to firing. While the apparatus of the present invention is set forth herein in conjunction with an AR-15 or M16 manufactured by Colt Industries, it is to be understood that the invention is useful in adapting any single-stage trigger assembly of similar automatic or semi-automatic weapons to a two-stage trigger. As used herein, the term "single-stage trigger" refers to a trigger having a constant trigger pull force, whereas a "two-stage trigger" has first and second stages with different trigger pull forces, the second typically being greater than the first. The AR-15 Sporter rifle has been offered for civilian sale in a semi-automatic-only configuration. The AR-15 Sporter is mechanically similar to the M-16, except that the necessary features to produce fully-automatic fire have been deleted, and the trigger housing modified to prevent conversion to automatic fire.
Many military weapons since 1900 have utilized a two-stage trigger system, where the shooter "feels" a slight change in trigger force immediately prior to discharge of the weapon. Such two-stage triggers were included on a number of bolt action models such as the 1917 Enfield, 1903 Springfield, the 30-40 Kraig and the Mauser. Later versions of non-bolt action models having two-stage triggers included the M-1 Garand and the M-14.
Two-stage triggers are useful for indicating to the shooter when the weapon is about to discharge. Such indication is necessary for optimal performance in both military and civilian use. For example, a sniper or a competitive target shooter must know when the weapon is about to fire in order to stabilize the weapon for an accurate shot. Without a two-stage trigger, the shooter has no way of knowing when the weapon will discharge as the trigger is moved or pulled.
It is the purpose of this invention to provide an adapter that can be placed within the trigger housing portion of a weapon having a single-stage trigger in order to create a two-stage trigger. The adapter is easily manufactured, easily installed or removed, and requires no re-working of the weapon into which it is placed.
It is a further object of the present invention to modify a weapon without adversely affecting the inherent safety of the weapon resulting from a large sear surface engagement between the trigger and the hammer.